Tires are typically manufactured on a cylindrical tire building drum with the tire components assembled in layers upon the drum. The carcass or load carrying member of the tire is typically made from one or more layers of ply which are cut and then spliced upon the tire building drum. The ply fabric is formed from a plurality of reinforcements which are coated in rubber prior to application to the drum.
In the early 1900s, bias tires were made of two or more layers of ply wherein the reinforcements were angled with the circumferential direction. The cord angle of the ply layer provided reinforcement in both the radial and circumferential direction. One advantage to bias tires is that since the cords are oriented at an angle, they have strength in the circumferential direction and the radial direction. The disadvantage to bias tires is that the cords of the ply were not placed in the most efficient path possible, resulting in energy loss.
In the 1970s, the radial tire became the industry standard. For the radial ply, the cords have a 90 degree angle with the circumferential direction, so that the cords are normal to the bead, running from bead to bead. One advantage to radial tires is that the cords are oriented efficiently, i.e., the shortest distance between two points. One disadvantage to radial tires is that they have low strength in the circumferential direction. Thus as a radial tire rolls through its contact patch, the tire bulges due to the lack of strength in the circumferential direction. The tire bulge is a source of energy loss which results in increased rolling resistance.
The next generation tire or tire of the future will most likely be a low rolling resistance tire due to the consumer demand for more fuel efficient vehicles. The inventors of this application have discovered that a geodesic tire could represent a viable solution to a low rolling resistance tire due to its unique properties. Geodesic tires are tires whose ply cord paths are geodesic lines on the tire surface, conforming perfectly to the geodesic law for an axisymmetric surface that ρ cos α=ρ0 cos α0. The result of the geodesic path is that the cord tension is uniform over the entire cord path, and that shear stresses due to inflation pressure are zero. A true geodesic path is the shortest distance between two points on a surface.
While the math of geodesic tires is described by Purdy, efforts to build a true geodesic tire have been elusive. Most of the efforts have been focused on building a geodesic tire flat on a tire building drum so that the cords would pantograph into the geodesic position upon formation into the final tire shape. This approach has not been proven to result in a geodesic tire. Thus for the foregoing reasons, it is desired to provide an improved method and apparatus for forming a geodesic tire without the above described disadvantages.